In drivetrains of so-termed parallel hybrid vehicles known from practice, which usually comprise in each case an internal combustion engine, an electric machine and a transmission device, thrust torque to be applied to the drive outputs is provided by the electric machine and the combustion engine. During this the electric machine is operated as a generator and electrical energy is recuperated. The electric machines, among other things, are arranged in the force flow between the combustion engine and the transmission devices of the drivetrains. In addition, between the electric machines and the drive outputs of the drivetrains in each case a shift element with continuously variable transmission capacity is provided, so that the nominal output torques to be applied at the drive output depends on the transmission capacity of the shift elements.
To save fuel, depending on the operating condition the internal combustion engines are shut off during thrust operation. To do this, the still running combustion engines and the electric machines are decoupled from the drive output by the shift elements and the combustion engines are then stopped actively by the electric machines.
In this, however, it is a disadvantage that the above-described decoupling of the combustion engines and electric machines gives rise to traction force interruptions. Furthermore, the above-described mode of operation reduces the spontaneity of the drivetrain to an undesired extent, since the time span between the specification of a driver's wish calling for a change in the operating condition of the drivetrain, and the time when the operating point of a drivetrain called for by the driver's wish specification has been reached, is quite long, so that too much time is taken until the drivetrain reacts to the changed driver's wish.
Other variants of drivetrains for parallel hybrid vehicles are formed, in the area between the internal combustion engine and the electric machines, in each case with a shift element or a separator clutch, by means of which the combustion engines can be decoupled from the drivetrains in these areas. In drivetrains of such design, whenever the combustion engines need to be shut off, they are first decoupled by the separator clutches from the drivetrains and then stopped.
However, this procedure disadvantageously results in a decrease of the driving comfort since the stopping of a combustion engine in the vehicle, in itself, excites uncomfortable oscillations which bring about vibrations of the vehicle. These oscillations result from irregularity of the running of the combustion engines during their running-down process. Furthermore, during run-down processes no thrust torques produced by the combustion engines for generating nominal output torques at the drive output are available when the separator clutches are open, since in the decoupled condition of the combustion engines their rotation energy cannot be utilized.